Method of making glass-to-metal seals



.of. the glass.

Patented Mar. 2, 1954 r iumrsio smrsrs PATENT OFFICE METHOD. OFIMAKING GLASS-T O-METAL SEALS Edwiu'tSmith', :Shdflield, England, assignor to 'Firtlr-Vickers Stainless Steels Limited, .Sheffield, England, a British company No Drawing. Application May 17,.1951,

Serial No. 226,937

Glaims-priority,xapplication Great Britain.

August 22,. 1950 stress. If, however; thisdifierence is too. great,

cracking-or strippingof the glassmay occur.

The present invention is particularly (but not exclusively) concerned with the sealing of cer-' tain alloys to relatively soft glasses having coefficients of thermal expansion in the region of .0000080 to .000010 per degree centigrade over the range 20 to 500 C. Such glasses are common glass as used, for example, in the manufacture of plate glass having a coefficient of expansion of the order .00000895 per degree centigrade, or another with an expansion coefficient of .00000930 per degree centrigrade, or a special high expansion glass with an expansion coefficient of .000009'72.

For the purpose of sealing to glasses of the above kind, ithas hitherto been a general practice to employ chromium irons containing 16 to 30 per cent of chromium. A drawback to the use of such alloys with a chromium content in the higher part (say 2630%) of the range, is the diihculty of producing the alloys in suitable form for subsequent use, since they have poor hot and cold working properties, in particular, they are difficult to spin or deep press. Efforts have been made to overcome these difficulties by various alloy additions, but these are only partially successful. In consequence, attention has been given to the alloys within the lower part or the chromium range, say 16 to 22% chromium, which are much'easier to work. These, however, have a slightly higher coefficient of expansion than the 26 to 30% alloys and can only be used with reasonable success for scaling to special glasses having high coefiicients of expansion. They also tend to suffer from transformation changes which have a deleterious effect on the sealing characteristics. One way of dealing with these transformation changes is to add stabilizing elements such as titanium, niobium, tantalum, aluminium, molybdenum, vanadium and tungsten, which suppress the transformation changes, or by using an alloy with a very low carbon content. ,"Ihesemodifications to the plain chromium iron alloys, however, do not materially 'afiect the normal thermal expansion rateand unless glasses with a suitable rate of expansion .are selected trouble still arises by cracking. or stripping of the glass.

The present .invention provides a. method of 'making a glass-to-metal seal which i character- .ised by the step of applying a light coating of cobalt or cobalt-containing substanceto the surface of the metal or the glass. in the vicinity of the seal, prior tov sealing. In making a seal by a method involving the coating step .aioresaid, the cobalt of the coating will oxidise to form cobalt oxide which will be absorbed by the hot plastic glass in its'vicinity', the thus modified glass being plastic at a much lower temperature than glass without cobalt oxide addition. In this way the method results in the production of a seal which is substantially stress-free and adherent.

While the invention may be applied to glassto-metal seals in general, the principal application which is contemplated is to seals between chromium iron alloys and relatively soft glasses of the kind previously indicated.

In carrying out the invention any convenient method may be employed for applying the cobalt coating. Electroplating or metal spraying methods may for example be used. Alternatively the coating may be applied by dusting the metal or glass surfaces with powdered cobalt oxide or powdered cobalt-containing substances. Again, if desired, the coating may be applied by painting or spraying the metal or glass with a suspension in liquid (e. g. Water, paraifin or oil) of cobalt oxide or cobalt-containing substances, in which case the liquid should be dried oii before the seal is made in order to prevent bubble formation in the glass.

The above methods are equally suitable for both types of plate glass available in this country and with both 17% chromium iron and 20% chromium iron available as standard products. The invention has also proved satisfactory with chromium irons containing appreciable amounts of retained austenite when cooled from the sealing temperature. Ihese materials would not otherwise give satisfactory scale.

The invention is particularly advantageous in the manufacture of spun cones of cathode-ray tubes used in the Television Industry, and in similar applications.

The invention is particulary applicable to the making of glass-to-metal seals using chromium iron comprising carbon 0.2% maximum, chromium to 30% and the remainder substantially all iron. The preferred range for the chromium iron is carbon 0.1% maximum, chromium 16-22% and the remainder substantially all iron. Stabilizing elements such as titanium, niobium, tantalum, aluminium, moylbdenum, vanadium and tungsten may also be present.

The invention includes glass-to-metal seals (e. g. for electric filament lamps, cathode-ray tubes and the like) when made by methods which include the characteristic step herein described.

I claim:

1. A method of making a glass-to-metal seal which consists in first applying a light coating consisting of metallic cobalt to at least one of the glass and metal surfaces at which the seal is to be formed, and then efiecting heat sealing under conditions such that the cobalt of the coating will oxidise to form cobalt oxide which will be absorbed by hot plastic glass in the vicinity,

the thus modified glass being plastic at a much lower temperature than glass without cobalt oxide addition.

2. A method of making a seal between glass and a chromium-iron alloy, which method consists in first applying, by the process of electro-plating,

a light coating comprising cobalt to the metal surfaces at which the seal is to be formed, and then effecting heat sealing under conditions such "that the cobalt of the coating will oxidise to form cobalt oxide which will be absorbed by the hot plastic glass in the vicinity of the seal, the thus modified glass being plastic at a much lower temperature than glass without cobalt oxide ad-- dition 3. A method of making a glass-to-metal seal between a chromium-iron alloy containing a portion of carbon 0.1 maximum, chromium 1622 and the remainder substantially all iron, and a relatively soft glass having a coeificient of thermal expansion in the region of .0000080 to .000010 per degree centigrade over the range 20 to 500 C,. which method consists in first applying a light coating comprising a cobalt containing substance which will upon heating form cobalt oxide to at least one of the glass and chromium-iron surfaces at which the seal is to be formed, and then effecting heat sealing under conditions such that from the coating aforesaid there will be produced cobalt oxide which will be absorbed by hot plastic glass in the vicinity of the seal, the thus modified glass being plastic at a substantially lower temperature than the same glass without such modification.

EDWIN SMITH.

References Cited in the file of this patent UNITED STATES PATENTS Great Britain June 16 1939 

1. A METHOD OF MAKING A GLASS-TO-METAL SEAL WHICH CONSISTS IN FIRST APPLYING A LIGHT COATING CONSISTING OF METALLIC COBALT TO AT LEAST ONE OF THE GLASS AND METAL SURFASCES AT WHICH THE SEAL IS TO BE FORMED, AND THEN EFFECTING HEAT SEALING UNDER CONDITIONS SUCH THAT THE COBALT OF THE COATING WILL OXIDES TO FORM COBALT OXIDE WHICH WILL BE ABSORBED BY HOT PLASTIC GLASS IN THE VICINITY, THE THUS MODIFIED GLASS BEING PLASTIC AT A MUCH LOWER TEMPERATURE THAN GLASS WITHOUT COBALT OXIDE ADDITION. 